this is clearly a mismatch of native vlan in cisco terms. the cisco device is expecting untagged packets to be packets destined to vlan 10 and sending out untagged packets that came from vlan 10 and when the Foundry rx'd this untagged packet it will not send those packet out to vlan 10 thus vlan 10 is not working. If you are only interested in making this work, make the natvie vlan in 3550 to be anything else otehr than vlan 10, preferrably a non-working vlan or dummy vlan, for example vlan 99. Try it let me know if you are able to ping in vlan 10 between the foundry and the cisco.

Why wouldn't I be able to ping the VLAN 10 interface on the 3560? The two ports are trunked now with VLAN 101 as an allowed VLAN so it should be layer two across that trunk. But I still can't reach the IP address for VLAN 10.

If you configured port as trunk on both sides, what's the native vlan? By default, Cisco switch won't tag native vlan in trunk. Did you permit vlan 10 on the trunk? Is your ping across vlan or just within one vlan?

On the Cisco side VLAN 1 & 10 are allowed across the trunk. On Foundry VLAN 10 is allowed in the trunk. I learned this morning that tagging (Foundry trunk term) doesn't apply to the default VLAN and now I just learned it doesn't apply to the default VLAN in Cisco either.

this is clearly a mismatch of native vlan in cisco terms. the cisco device is expecting untagged packets to be packets destined to vlan 10 and sending out untagged packets that came from vlan 10 and when the Foundry rx'd this untagged packet it will not send those packet out to vlan 10 thus vlan 10 is not working. If you are only interested in making this work, make the natvie vlan in 3550 to be anything else otehr than vlan 10, preferrably a non-working vlan or dummy vlan, for example vlan 99. Try it let me know if you are able to ping in vlan 10 between the foundry and the cisco.